Object Mentor Blog: Strongly Typed Languages Considered Dangerous

Strongly Typed Languages Considered Dangerous

Brett Schuchert — Thu, 23 Aug 2007 18:37:00 -0500

Have you ever heard of covariance? Method parameters or returns are said to be covariant if, as you work your way down an inheritance hierarchy, the types of the formal parameters or return type in an overridden method can be sub-types of the formal parameters and return types in the superclass’ version of the method.

Oh, and contravariance is just the opposite.

What?! Why should you care? Answer, you shouldn’t. Yes C++ and Java both support covariant return types, but so what? Have you ever used them? OK, I have, but then I also used and liked C++ for about 7 years, over 10 years ago. We all learn to move on.

You ever notice how something meant to help often (typically?) turns out to do exactly the opposite? Even worse, it directly supports or enables another unfortunate behavior. This is just Weinberg’s first rule of problem solving:

Every solution introduces new problems

Here’s an example I’m guilty of. Several years ago I was working on a project where we had written many unit tests. We had not followed the FIRST principles, specifically R-Reliability. Our tests were hugely affected by the environment. We had copies of real databases that would get overridden without warning. We’d have problems with MQ timeouts at certain times of the day or sometimes for days. And our tests would fail.

We wold generally have “events” that would cause tests to fail for some time. We were using continuous integration and so to “fix” the problem, I created a class we called “TimeBomb” – turns out it was the right name for the wrong reason.

You’d put a TimeBomb in a test and then comment out the test. The TimeBomb would be given a date. Until that date, the test would “pass” – or rater be ignored. At some point in the future, the TimeBomb would expire and tests would start failing again.

I was so proud of it, I even took the time to write something up about it here. I had nothing but the best intentions. I wanted an active mechanism to remind us of work that needed to be done. We had so many todo’s and warnings that anything short of an active reminder would simply be missed. I also wanted CI to “work.” But what eventually happened was that as our tests kept failing, we’d simply keep moving the TimeBomb date out.

I wrote something that enabled our project to collect more design debt. As I said, my intentions were noble. But the road to Hell is paved with good intentions. Luckily I got the name right. The thing that was really blowing up, however, was the project itself.

What has all of this got to do with “strongly typed languages”?

If you’re working with a strongly typed language, you will have discussions about covariance (well I do anyway). You’ll discuss the merits of multiple inheritance (it really is a necessary feature – let the flames rise, I’m already in Hell from my good intentions). You’ll also discuss templates/generics. The list goes on and on.

If you’re working with a dynamically typed language (the first one I used professionally was Smalltalk but I also used Self, Objective-C, which lets you choose, and several other languages whose names I do not recall).

Covariance is not even an issue. The same can be said of generics/templates and yes, even multiple inheritance is less of an issue. In a strongly-typed languages, things like Multiple Inheritance are necessary if you want your type system to be complete. (If you don’t believe me, read a copy of Object Oriented Software Construction, 2nd ed. by Bertrand Meyer – an excellent read.)

Ever created a mock object in a typed language? You either need an interface or at least a non-final class. What about Ruby or Smalltalk? Nope. Neither language cares about a class’ interface until it executes. And neither language cares how a class is able respond to a particular message, just that it does. It’s even possible in both languages to NOT have a method and still work if you mess with the meta-language protocol.

OK, but still, does this make typed languages bad?

Lets go back to that issue of enabling bad things.

Virtual Machines, like the JVM and CLR, have made amazing strides in the past few years. Reflection keeps getting faster. Just in time compilers keep getting smarter. The time required for intrinsic locks has improved and now a Java 5 compiler will support non-blocking, safe, multi-threaded updates. Modern processors support such operations using an optimistic approach. Heck, Java 6 even does some cool stuff with local object references to significantly improve Java’s memory usage. Java runs pretty fast.

Dynamic languages, generally, are not there yet. I hope they get there, but they simply are not there. But so what?! If your system runs “fast enough” then it’s fast enough. People used Smalltalk for real applications years ago – and still do to some extent. Ruby is certainly fast enough for a large class of problems.

How is that? These languages force you to write well. If you do not, then you will write code that is not “fast enough.” I’ve see very poor performing Smalltalk solutions. But it was never because of Smalltalk, it was because of poor programming practices. Are there things that won’t currently perform fast enough in dynamically typed languages? Yes. Are most applications like that? Probably not.

You can’t get away with as much in a dynamically typed language. That sounds ironic. On the one hand you have amazing power with dynamically typed languages. Of course Peter Parker learned that “With great power comes great responsibility.” This is just as true with Ruby and other dynamically typed languages.

You do not have as much to guide you in a dynamically typed language. Badly written, poorly organized code in a typed language is hard to follow but it’s possible. In a language like Ruby or Smalltalk it’s still possible but it’s a lot harder. Such poor approaches will typically fail sooner. And thats GOOD! You’ve wasted less money because you failed sooner. If you’ve got crappy design, you’re going to fail. The issue is how much time/money will you spend to get there.

Another thing that strongly typed languages offer is a false sense of security because of compiler errors. I have heard many people deride the need for unit tests because the compiler “will catch it.”

This misses a significant point that unit tests can serve as a specification of behavior rather than just a validation of as-written code.

You cannot get away with as much in a dynamically typed language. Or put another way, a dynamically typed language does not enable you to be as sloppy. It’s just a fact. In fact you can typically get away with a lot in a dynamically typed language, you just have to do it well.

Does this mean that dynamically typed languages are harder to work in? Maybe. But if you follow TDD, then the language is less important.

Do we need fast, typed languages? Clearly we do. There are applications where having such languages is necessary. Device drivers are not written in Ruby (maybe they are, I’m just trying to be more balanced).

However, how many of you were around when games were written in assembly? C was not fast enough. Then C was fast enough but C++ was still a question mark. C++ and C became mainstream but Java was just too slow. Some games are getting written in Java now. Not many, but it’s certainly possible. It is also possible to use multiple languages and put the time-critical stuff, which is generally a small part of your system, in one language and use another language for the rest of the system.

Back in the very early 90’s I worked just a little bit with Self. At that time, their Just In Time compiler would create machine code from byte code for methods that got executed. They went one step further, however. Not only did they JIT compile a method for an object, they would actually do it for combinations of receivers and type parameters.

There’s a name for this. In general it is called multi-dispatch (some languages support double-dispatch, the visitor pattern is a mechanism to turn standard polymorphism into a weak form of double dispatch, Smalltalk used a similar approach for handling numerical calculations).

Self was doing this not to support multi-dispatch but to improve performance. That means that a given method could have multiple compiled forms to handle commonly used methods on a receiver with commonly-used parameters. Yes it used more memory. But given what modern compilers do with code optimization, it just seems that this kind of technique could have huge benefits in the performance of dynamically typed languages. This is just one way a dynamic language can speed things up. There are others and they are happening NOW.

I’m hoping that in the next few years dynamic languages will get more credit for what they have to offer. I believe they are the future. I still primarily develop in Java but that’s just because I’m waiting for the dust to settle a little bit and for a clear dynamic language to start to assert itself. I like Ruby (though its support for block closures is, IMO, weak). I’m not convinced Ruby is the next big thing. I’m working with it a little bit just in case, however.

What are you currently doing that enables yourself or your co-workers to maintain the status quo?

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